Mineral Dust and its Microphysical Interactions with Clouds - Mineral Dust: A Key Player in the Earth System - page 307

Fig. 12.8 Experimentally derived ice nucleation onset temperatures and saturation ratios for
natural and surrogate for natural dusts, Arizona Test Dust ( ATD ) as presented in the review of
Hoose and Möhler ( 2012 ). SD Saharan dust, AD Asian dust, CID Canary Islands dust, ID Israeli
dust
apart from differences in mineralogy discussed above, could be that the surfaces of
natural dusts are chemically aged or coated, whereas for ATD the grinding process
reveals fresh mineral surfaces on which ice can nucleate more effectively. In fact,
the role of coatings on mineral dusts has been explored in the laboratory in the past.
It has been shown in a number of experimental studies that exposure of mineral
dusts to inorganic and organic coatings can reduce their ice-nucleating ability
(Hoose and Möhler 2012 ). Reaction with sulfuric acid has received significant
attention and studies have focused on both ice nucleation below water saturation,
which is most relevant for cirrus clouds (Chernoff and Bertram 2010 ; Cziczo et al.
2009 ; Knopf and Koop 2006 ; Archuleta et al. 2005 ; Eastwood et al. 2009 ), and also
with particles suspended inside water droplets at temperatures pertinent for mixed-
phase clouds (Niedermeier et al. 2011a ; Sullivan et al. 2010a , b ; Wex et al. 2013 ).
It has become apparent that the effect of the coating depends on the nucleation
pathway and the nature of the coating material. Tobo et al. ( 2012 ) showed that both
chemically inert levoglucosan and reactive sulfuric acid coatings reduced the ice-
nucleating ability of kaolinite particles below water saturation, whereas at water
saturation, only sulfuric acid-treated particles were less active. This shows that
below water saturation, the pathway of nucleation changes from deposition on bare
mineral to nucleation of ice by particles immersed in solution droplets and that this